Lately, it is strongly demanded to reduce a fuel consumption of an automobile and hence a tire having a lower rolling resistance is required. For this end, there is required a rubber composition being excellent in the low loss factor (low heat buildup) as a rubber composition used in a tread or the like of the tire. Also, the rubber composition for the tread is required to be excellent in the wear resistance and fracture characteristics in addition to the low loss factor. As to these requirements, in order to improve the low loss factor, wear resistance and fracture characteristics of a rubber composition formed by compounding a filler such as carbon black or the like into a rubber component, it is effective to improve an affinity between the filler and the rubber component in such a rubber composition.
For example, in order to improve the reinforcing effect with the filler by improving the affinity between the filler and the rubber component in the rubber composition, there are developed a synthetic rubber wherein the affinity for the filler is improved by a terminal modification, a synthetic rubber wherein the affinity for the filler is improved by copolymerizing with a functional group-containing monomer and so on.
On the other hand, a natural rubber is voluminously used while utilizing its excellent physical characteristics, but there is no technique wherein the affinity for the filler is improved by modifying the natural rubber itself to highly improve the reinforcing effect with the filler.
For example, there is proposed a technique for epoxidizing the natural rubber, but the affinity between the natural rubber and the filler cannot be sufficiently improved in this technique, so that the reinforcing effect with the filler cannot be sufficiently improved (for example, see JP-A-H06-329702). Also, there is known a technique wherein a graft-polymerization is conducted by adding a vinyl-based monomer to a natural rubber latex, and the grafted natural rubber obtained by this technique is put into a practical use as an adhesive or the like (for example, JP-A-H09-25468 and JP-A-2002-138266). In the grafted natural rubber, however, a large amount of the vinyl compound as a monomer (20-50% by mass) is grafted for changing the characteristics of the natural rubber itself, so that when it is compounded with carbon black, the viscosity is largely increased and the processability is deteriorated. Moreover, since the large amount of the vinyl compound is introduced into the molecular chain of the natural rubber, there are damaged the excellent physical characteristics inherent to natural rubber (viscoelasticity, stress-strain curve in a tensile test and the like).
Moreover, JP-A-2004-359713 discloses that a rubber composition having improved wear resistance and low heat buildup is obtained with a modified natural rubber formed by graft-polymerizing a polar group-containing monomer onto a natural rubber molecule in a natural rubber latex.